Structure, Stability, Spectroscopy and Chemical Bonding Analyses on the Novel X ₆ (X = N, P, As, Sb, Bi) Molecules

ABSTRACT

Inspired by the successful synthesis of zig‐zag C _2h ‐N ₆ ( Nature 2025 , 642, 356–360), we systematically explored the structure, stability, infrared spectroscopy, chemical bonding and weak interaction of novel X ₆ (X = N, P, As, Sb, Bi) molecules using relativistic DFT and CCSD(T) method. The results of global‐minimum structural searches indicate the N ₆ prefers a trimer‐like C ₃ geometry, while P ₆ /As ₆ tends to form the bag‐like C _2v structure, and Sb ₆ /Bi ₆ prefers the triangular prism D _3h structure energetically instead. Simulated infrared spectra reveal the trimer‐like N ₆ exhibiting an isolated high‐frequency triple‐bond stretching band far separated from the low‐frequency single‐bond skeletal vibration regimes of heavier pnictogen hexamers. Canonical molecular orbital analyses reveal a non‐monotonic periodic evolution of HOMO–LUMO electronic band gaps, with N ₆ , Sb ₆ and Bi ₆ possessing substantially wider frontier orbital energy separation relative to narrow‐gap bag‐like P ₆ and As ₆ species. Multi‐modal wavefunction analytical tools including AdNDP, QTAIM, ELF, IRI and RDG/NCI resolve continuous periodic evolution of covalent bonding motifs from discrete diatomic triple bonds in N ₆ , mixed σ and π bonding in P ₆ and As ₆ , to pure localized σ single‐bonded frameworks devoid of π orbital contributions in Sb ₆ and Bi ₆ . Quantified dissociation energy values for two representative uni‐molecular fragmentation pathways demonstrate monotonically increasing resistance to full decomposition from N ₆ to Bi ₆ . Additional SOC‐CASSCF energy level calculations further quantify the progressive intensification of spin‐orbit coupling relativistic effects down group 15, which exert negligible electronic perturbation on N ₆ and P ₆ , moderate orbital splitting for As ₆ , and decisive rearrangement of frontier orbital manifolds for Sb ₆ and Bi ₆ that dictates geometric stability preferences of heavy hexamer prism. This work systematically reveals the periodic evolution law of group VA X ₆ molecules, providing key theoretical insights for the design and synthesis of novel pnictogen clusters.